The Viral Analysis Unit of the LMMN has evaluated over 2000 clinical samples including CSF, plasma and serum for the detection of JC Virus genome sequences. This finding is diagnostic for PML, a rare but mostly fatal disease of the brain. The LMMN receives many such samples from around the world in circumstances in which diagnosis cannot be made on clear clinical findings alone. The VAU has made humdreds of determinations on such samples and provided critical information to neurologists and their patients for future medical care and treatment. In a survey of hundreds of plasma samples, it became clear that over 2% of the population worldwide can be viremic and that approximately 1% of the population is persistently viremic. Such numbers in a well defined and reliable assay system. The nature of the treatment that such viremic patients may receive for underlying diseases particularly that are classified as autoimmune diseases i.e. Multiple Sclerosis, Rheumatoid arthritis, Crohn's disease, Systemic Lupus Erythymatosis. Of the 65 MS patients that developed PML while treated with natalizumab,48 were diagnosed with the data from the Viral Analysis Unit. There have been at least 6 of these patients that had been misdiagnosed and not given correct medical treatment until the VAU provided the correct evidence of PML